The invention relates to a two-pole step motor for an analog electronic timepiece.
Since an analog electronic timepiece employs a cell for its power source, the analog electronic timepiece stops its function after continuous operation for a given length of time period due to exhaustion of its capacity. Accordingly, the cell need be replaced periodically with a new one, which has been quite troublesome to users.
Further, as the users have to ask specialist shops to do such replacement, it has been impossible to have the cell replaced immediately if the cell runs down when there is a need for use of the analog electronic timepiece, and consequently, this has caused a great deal of inconvenience to the users.
Since such exhaustion of the capacity of the cell of the analog electronic timepiece poses a major problem to the users, efforts have been made lately to study on prolongation of a service life of the cell in the analog electronic timepiece or to develop a timepiece capable of eliminating a need for replacement of a cell by incorporating a generator in the timepiece, activated following the motion of the users carrying the timepiece with them, or by the agency of a power generation mechanism such as a solar cell, and so forth, incorporated in the timepiece.
However, in the case of an analog electronic timepiece with such a power generation mechanism built therein, the timepiece is designed to be driven by power stored in a capacitor or a secondary cell built therein, however, there have been cases where it has been difficult to generate sufficient power as required all the time because application conditions of the timepiece varies from one user to another.
Accordingly, even with the timepiece incorporating the power generation mechanism built therein, it has been necessary to aim at achievement of lowering power consumption in order to keep the timepiece in a stable operational condition without interruption during usage.
Meanwhile, if use can be made of a cell which is large in size, having a large capacity, it is possible to achieve prolongation of the service life thereof, however, designing constraints imposed on a timepiece does not permit the cell to be excessively large in size. Accordingly, if prolongation of the service life of the cell is called for, it has been inevitable to achieving lowering of power consumption on the part of the timepiece.
Now, a mechanism of an analog electronic timepiece is broadly described hereinafter. It has a construction such that a two-pole step motor for a timepiece is intermittently driven in accordance with a reference signal generated by a quartz oscillator, and the like, and time display is performed by transmitting motion of the step motor to the hands of the timepiece via gears.
It follows therefore that from the viewpoint of power consumption, such an analog electronic timepiece can be broadly broken down into a circuit part incorporating the quartz oscillator and the like for generating the reference signal, and a step motor part for rotating the hands of the timepiece.
However, with analog electronic timepieces in current use, a circuit part is made up of a semiconductor integrated circuit wherein power consumption is rendered small, and consequently, a greater part of power is after all consumed for driving the step motor for handling the hands. Accordingly, reduction in power consumption of the step motor has a considerable effect on lowering of power consumption of a timepiece in whole.
FIG. 22 is a plan view showing a schematic construction of a conventional two-pole step motor for a timepiece.
The two-pole step motor for a timepiece (referred to hereinafter merely as xe2x80x9cstep motorxe2x80x9d) comprises a field coil 7 provided with a conductor 7b wound around a magnetic core 7a formed of a high-permeability material, and a stator 201 bonded to opposite ends of the magnetic core 7a of the field coil 7 by screws 8, 8, respectively, for magnetic connection.
The stator 201 is provided with a rotor hole 202 defined substantially at the center thereof, and a rotor 3 is rotatably disposed inside the rotor hole 202.
Further, the rotor 3 is comprised of a rotor magnet 3a and a rotor axle 3b, and the rotor magnet 3a is made of a ferromagnetic material and is formed in a low-profile columnar shape. The rotor axle 3b serving as a rotation axis is inserted into an axle hole defined at the center of the rotor magnet 3a in the direction normal to the plane of the figure so as to be integrally joined together, thereby magnetizing the rotor magnet 3a in such a way as to have two poles in the diametrical direction thereof.
The rotor 3 with opposite ends of the rotor axle 3b rotatably supported by bearings (not shown), respectively, is positioned at the center of the rotor hole 202. Further, the rotor 3 is constituted such that a gear is provided at one end of the rotor axle 3b, and rotatory motion thereof is transmitted via the gear to the hands of the timepiece.
Further, holding torque setting means is provided on the inner periphery of the rotor hole 202, so that the magnetic poles of the rotor magnet 3a are positioned so as to be oriented in a constant direction of an initial phase angle xcex81 by the agency of the holding torque setting means when the step motor is out of operation, thereby stopping and holding the rotor 3 in that position with a predetermined holding torque.
With the step motor, by applying a driving voltage thereto, forward and reverse current are caused to flow alternately through the field coil 7, thereby a magnetic field oriented in a direction corresponding to the direction of the forward and reverse current, respectively, is generated inside the rotor hole 202 so as to correspond to the magnitude of the respective flowing current, and the magnetic field is caused to act on the rotor magnet 3a magnetized beforehand, so that the rotor 3 is rotated by 180 degrees (for one step) counterclockwise in FIG. 22.
The motion of the step motor, made for one step, is described hereinafter.
If the direction of a magnetic field produced inside the rotor hole 202 by magnetic fluxes which are generated when current is caused to flow through the field coil 7 is designated as an excitation direction line 12, the rotor 3 is held and stopped at a position where a line 4, which is the direction of magnetization of the rotor magnet 3a, and which interconnects the two poles thereof, is rotated by the initial phase angle xcex81 counterclockwise in FIG. 22, relative to the excitation direction line 12, by the agency of the holding torque of the holding torque setting means, established by magnetic action between the magnetic poles of the rotor magnet 3a and the stator 201 in a state where no current flows through the field coil 7.
In this state, when current is caused to flow through the field coil 7 in such a direction as to cause the rotor 3 to rotate forward, magnetic fluxes occur to the field coil 7, and a magnetic field is generated inside the rotor hole 202, whereupon the rotor 3 is subjected to a rotational torque caused by an interaction of the magnetic field and the permanent magnetized charge of the rotor magnet 3a, starting rotation against the resistance of the holding torque. Upon flowing of current through the field coil 7 for a suitable duration only, the rotor 3 stops after being rotated through 180xc2x0 up to a position of the next stop.
With the step motor of the constitution as described above, power consumption for a unit of time is expressed by the product of a strength of the current caused to flow through the filed coil 7 for excitation, and a cell voltage as applied. Since the cell voltage as applied in this case remains substantially constant, lowering of the power consumption of the step motor depends on how to reduce current flowing in the field coil 7 while satisfying driving characteristics required of the step motor.
Further, with the step motor, the rotational torque is caused to occur to the rotor 3 by causing current to flow in the field coil 7, thereby causing the rotor 3 to rotate against the resistance of the holding torque. Consequently, the smaller the holding torque, the smaller the rotational torque as required may be in proportion to the holding torque.
Since current which is caused to flow in the field coil 7 is proportional to the rotational torque, current flowing in the field coil 7 can be reduced if the holding torque can be reduced. As a result, it becomes possible to achieve lowering of power consumption of the step motor for a timepiece.
Now, the holding torque of the step motor for a timepiece has functions such that even when the timepiece is subjected to an impact when the timepiece is dropped, and so forth, the hands are securely held so as not to be caused to jump, thereby enabling correct time to be displayed while settling the hands at a correct stop position against the resistance of friction torque occurring to bearings and gears inside the timepiece.
Accordingly, it is not as simple as a case where the holding torque need only be rendered smaller in order to reduce power consumption, but it is required that the holding torque be set so as to meet the minimum holding torque as required to maintain the function of the timepiece.
As disclosed in International Publication No. WO 98/30939, it is described with reference to the holding torque as required for use in timepiece that jumping of the hands will not occur if kinetic energy occurring to the hands by an impact is smaller than a holding potential established by the holding torque of a rotor, that is, a magnetic potential difference.
Since kinetic energy received by the hands when subjected to the impact is proportional to the square of moment of the hands, the holding potential, that is, the holding torque can be rendered smaller by use of the hands with a smaller moment.
By so doing, it becomes possible to set the minimum holding torque as required at a very small value equivalent to a fraction of the holding torque of a step motor for a timepiece, thereby achieving lowering of power consumption of the timepiece.
Next, holding torque setting means, provided in a stator of the conventional step motor for a timepiece, is now described hereinafter.
As for the holding torque setting means, provided in the stator of the conventional step motor for a timepiece, there are primarily two types in construction as described below.
One type has a construction such that the stator 201 of the step motor for a timepiece , shown in FIG. 22, is formed of a high-permeability material, and as shown in FIG. 23, there are provided holes 6, 6 defined close to opposite ends of the stator 201 in the longitudinal direction, for bonding the stator to opposite ends of the magnetic core 7a of the field coil 7.
A rotor hole 202 provided substantially at the center of the stator 201 is defined in the shape of two semicircles joined together with the center of the respective semicircles deviated from each other to permit a holding torque and an initial phase angle xcex81 (refer to FIG. 22) to be set.
By combining the two semicircles in such a way as to cause the center of the respective semicircles to deviate from each other, two stepped parts 204a, 204b having a gap amount G, respectively, are formed. With the stator 201, it is possible to set the holding torque to a desired value by adjusting the gap amount G.
The construction wherein such stepped parts 204a, 204b described above are formed inside the rotor hole 202 of the stator 201 is described in, for example, Japanese Patent Laid-open No. S 49-132507.
A stator wherein such stepped parts are formed inside a rotor hole thereof is hereinafter referred to as a gap type stator.
Next, the construction of another type of holding torque setting means is described hereinafter with reference to FIG. 24. In the figure, some components used in common is described where necessary by using the same reference numerals as described with reference to FIG. 22.
A stator 211 in this case is provided with a pair of recesses 205a, 205b formed at symmetrical positions against the center axis of the rotor hole 212 on the inner periphery of the rotor hole 212, as holding torque setting means in order to provide the holding torque and the initial phase angle of a rotor 3.
Further, a straight line 24 passing through the respective centers of the recesses 205a, 205b is disposed so as to be tilted at an angle of xcex811 relative to an excitation direction 21 of the rotor hole 212.
With the stator 211, an angle which the straight line 24 passing through the respective centers of the recesses 205a, 205b forms with a straight line 27 passing through the center axis of the rotor hole 212 and orthogonal to the excitation direction of the stator 211, is designated as an installation angle xcex812 of the recesses 205a, 205b expressed in a positive value when rotated in counterclockwise direction, and the initial phase angle xcex81 (refer to FIG. 22) of the rotor 3 is set by adjusting the installation angle xcex812.
In the case of the step motor for a timepiece , having the stator 211 of such a construction as described above, the holding torque of the rotor 3 is determined by the pair of the recesses 205a, 205b. 
In this connection, the construction wherein the recesses 205a, 205b as described above are formed inside of the rotor hole 212 of the stator 211 is described in, for example, Japanese Patent Laid-open No. S 51-1908.
A stator wherein recesses are formed inside a rotor hole thereof is hereinafter referred to as a notched type stator.
As described in the foregoing, in the case of the conventional step motor employing the gap type stator, the magnitude of the holding torque and the initial phase angle can be adjusted by varying the gap amount of the stepped parts formed inside the rotor hole.
With the ordinary step motor for a timepiece, since the diameter of the rotor hole is in the order of 1700 xcexcm on average, the maximum holding torque can be set to around 300 nNm by setting the gap amount of the stepped parts of a stator to about 40 to 50 xcexcm.
However, if it is intended to further reduce the holding torque to a large extent in order to achieve lowering of power consumption, the gap amount need be rendered to be extremely small, as small as about 10 xcexcm, and consequently, it becomes difficult in respect of precision with which to process the stator to establish a stable holding torque.
Further, if the gap amount is rendered to be extremely small as described above, this leads to resultant reduction in the initial phase angle (refer to xcex81 in FIG. 22). As a result, this will result in requirement for large power consumption when driving the rotor, so that lowering of power consumption can not be achieved.
Further, in the case of the step motor employing the gap type stator construction, it is possible to set the holding torque to a small value even at the same gap amount without varying the initial phase angle by enlarging the diameter of the rotor hole, however, such enlargement of the diameter of the rotor hole will result in reduction of interaction between a magnetic field occurring inside the rotor hole and the rotor magnet.
That is, in this case, as electromechanical coupling constant decreases, the rotational torque occurring to the rotor by flow of current through the field coil is reduced.
As a result, even if the initial phase angle is set to a proper value by lowering the holding potential established by the holding torque, it will become necessary to increase current flowing in the field coil to compensate for a decrease in the rotational torque due to a decrease of the electromechanical coupling constant, so that a power-saving effect resulting from the holding torque being set to a small value will be offset, thereby rendering it impossible to achieve lowering of power consumption.
Meanwhile, in the case of the step motor employing the notched type stator construction, the initial phase angle can be set by the installation angle of the pair of the recesses while the holding torque is adjusted by either increasing or decreasing the sum of areas of the recesses formed on the inner periphery of the rotor hole, and consequently, if it is intended to render the holding torque considerably less than the present value in order to lower power consumption, this will require the sum of the areas of the pair of the recesses, in other words, dimensions of the recesses to be rendered extremely small. Accordingly, it will become difficult in respect of precision with which to process the stator to obtain a stable holding torque.
Further, with the notched type stator as well, it is possible to set the holding torque to a small value without varying the sum of the areas of the recesses by enlarging the diameter of the rotor hole, however, as with the case of the gap type stator, such enlargement of the diameter of the rotor hole will result in a decrease of the electromechanical coupling constant, so that lowering of power consumption can not be achieved.
As described hereinbefore, with the stator of the conventional construction as described above, if it is intended to set the holding torque to a small value in an attempt to further reduce power consumption, it has been necessary to render either the gap amount of the stepped parts formed in the stator or the dimensions of the recesses formed in the stator to be extremely small, thus posing difficulty in respect of precision with which to process the stator. Accordingly, it has been difficult to set a stable holding torque.
Consequently, with the step motor for a timepiece, adopting the conventional construction, it has been difficult to achieve lowering of power consumption.
The invention has been developed against the technical background described above, and it is an object of the invention to solve the problems as described above by devising a novel construction of a stator, and to provide a two-pole step motor for a timepiece which is suitable for lowering of power consumption, and which can be manufactured with ease.
To achieve the above objects, a two-pole step motor for a timepiece according to the invention comprises: a rotor made up of a rotor magnet and a rotor axle; a stator made of a high-permeability material, having a rotor hole in which the rotor is installed; and a field coil for excitation, provided with a magnetic core made of a high-permeability material around which a conductor is wound, and opposite ends of which are magnetically bonded to opposite ends of the stator, wherein the stator is provided with a plurality of holding torque setting means, disposed on the inner periphery of the rotor hole, at installation angles differing in the direction of the inner periphery.
Herein, the installation angle of the holding torque setting means refers to an installation angle relative to the direction orthogonal to an excitation direction of the stator, and the installation angle that differs by 180xc2x0 is deemed to be an equivalent installation angle.
It is effective for attaining lowering of power consumption to set the initial phase angle xcex81 which is an angle formed by the magnetic field direction line in the direction of a magnetic field produced inside the rotor hole and the magnetizing direction line of the rotor magnet at the standstill position of the rotor based on respective installation angles of the plurality of the holding torque setting means, in a range of 50 degrees to 70 degrees.
With the two-pole step motor for a timepiece according to the invention, even in the case where a single holding torque setting means can not be installed at an installation angle required for obtaining an initial phase angle and a holding torque as intended owing to presence of the axle hole of gears or holes of fixed pins which are formed around the rotor hole of the stator, the initial phase angle and the holding torque as intended can be obtained by breaking down a holding torque established by a pair of the holding torque setting means into vectors, and by installing two or more holding torque setting means corresponding to the respective vectors as broken down, at different installation angles and at locations avoiding the axle hole and the holes of fixed pins.
Further, the stator is preferably made up by bonding a first stator part made of a high-permeability material to a second stator part made of a high-permeability material through the intermediary of connections made of a low-permeability material or a nonmagnetic material.
In such a case, the stator has a construction such that it is magnetically separated into two portions, and consequently, a magnetic field inside the rotor hole for rotating the rotor can be efficiently produced by magnetic fluxes excited by the field coil, so that current caused to flow in the field coil can be reduced, thereby attaining lowering of power consumption.
Further, since the connections are made of either a low-permeability material or nonmagnetic material, there is no need of narrowing down the connections to an extreme extent, thereby enabling mechanical strength as required to be secured.
Furthermore, with the two-pole step motor for a timepiece described above, it is preferably that the connections where the first stator part is bonded to the second stator part serve as at least one of the plurality of the holding torque setting means while other holding torque setting means except the connections are disposed on the inner periphery of the rotor hole at installation angles differing from that for the connections.
In addition, the plurality of the holding torque setting means are preferably paired recesses or paired protuberances formed on the inner periphery of the rotor hole, respectively. Further, the other holding torque setting means as described above are preferably a pair of recesses or a pair of protuberances formed on the inner periphery of the rotor hole, including means formed in a shape asymmetrical with respect to the center of the rotor hole.
Then, the holding torque can be adjusted by varying the dimensions of the recesses or the protuberances, and the initial phase angle can be adjusted by varying the installation position of the recesses or the protuberances.
Further, among the holding torque setting means, the means formed in the shape asymmetrical with respect to the center of the rotor hole may be a pair consisting of a recess and a protuberance facing each other, formed on the inner periphery of the rotor hole, on opposite sides of the center of the rotor hole, or may comprise a recess or a protuberance formed on the inner periphery of the rotor hole only on one side of the center thereof.
Furthermore, the plurality of the holding torque setting means are preferably combination of those of different types with the installation angles thereof in the direction of the inner periphery of the rotor hole differing from each other.
In this connection, the combination of those of different types among the holding torque setting means is preferably combination of the gap type and the notched type, described in the foregoing, or combination of an oval type as described hereinafter and the notched type described above.
Still further, with the two-pole step motor for a timepiece having the plurality of the holding torque setting means including the means formed in a shape asymmetrical with respect to the center of the rotor hole, the first stator part is preferably bonded to the second stator part through the intermediary of the connections made of a low-permeability material or nonmagnetic material, the connections serving as at least one of the plurality of the holding torque setting means.
Similarly, with the two-pole step motor for a timepiece having the plurality of the holding torque setting means of different types, the first stator part is preferably bonded to the second stator part through the intermediary of the connections made of a low-permeability material or nonmagnetic material, the connections serving as at least one of the plurality of the holding torque setting means.